The present disclosure relates to an integrated layer stack arrangement, an optical sensor and also a method for producing an integrated layer stack arrangement.
In a conventional CMOS camera (Complementary Metal Oxide Semiconductor camera), a plurality of layer stacks having alternately in each case a patterned metal layer, usually made of aluminum in accordance with the prior art, and an intermetal dielectric layer, usually made of silicon dioxide, are applied above a photodiode integrated in a substrate. Light to be detected which is radiated in onto the known CMOS camera passes through the plurality of layer stacks and finally impinges on the photodiode. In the photodiode, the optical energy is converted into electrical energy and the electrical energy is if appropriate amplified, digitized and processed further by means of an evaluation circuit.
In future CMOS technologies, the material used for the metal layers, i.e. the metallization planes, will no longer be aluminum but rather is expected to be copper. With the use of copper for wiring in the respective metallization planes, diffusion barriers are required in order to prevent copper atoms from diffusing into the intermetal dielectric, preferably silicon dioxide (SiO2). An alternative material for the intermetal dielectric is an electrically insulating low-k material, for example SiLK™. Silicon nitride (Si3N4) is usually used as material for the diffusion barriers.
In future products, the layer thickness of a diffusion barrier layer will be approximately 50 nm and less and the thickness of an intermetal dielectric layer will be approximately 400 nm to 1000 nm.
Silicon is usually used as a substrate. If a photodiode is integrated in the silicon substrate in the product, from the surface of the component a light that is incident there has to penetrate through all the layer stacks applied above the photodiode until it can penetrate into the photodiode and can lead to the generation of electrical charge carriers there.
Owing to the different optical density of the material of the intermetal dielectric (silicon dioxide/low-k material usually have a refractive index n of approximately 1.5) and of the diffusion barrier (silicon nitride has a refractive index n of approximately 2) multiple interferences occur in the optical path of the light beams passing through the layer stacks.
FIG. 2 shows an ascertained transmission curve 201, illustrating the transmission coefficient 202 depending on the wavelength 203 of the light that is respectively radiated in for an above-described layer stack arrangement in accordance with the prior art with copper as metal for the conductor tracks and silicon nitride as diffusion barrier material.
As can be gathered from the transmission curve 201 in FIG. 2, on account of the, to an approximation, chaotic and thus unpredictable distributions of the transmission curve maxima, it is not possible to suitably dimension the layer stack arrangement such that a maximum quantity of light impinges on the photo-diode, i.e. that only a minimum quantity of light is reflected by the layer stacks.
As can furthermore be gathered from the transmission curve 201, a plurality of layer stacks in the layer stack arrangement of a CMOS camera in the case where copper metallization is used reduces the mean optical transmission to 65% for individual wavelengths even down to 20%.
For a CMOS camera or for an optical mouse into which is introduced an electronic chip with above-described technology with photodiodes, this leads to an increased current consumption and to a reduction of the yield.
One solution to the problem described above might consist in optimizing the thicknesses of the layers of the intermetal dielectric and of the diffusion barriers and in monitoring the thicknesses of the layers very precisely and thus optimizing the transmission for a specific wavelength. However, this leads to a considerably increased outlay in the production of such an integrated layer stack arrangement.
One alternative would be, furthermore, to replace the material of the diffusion barrier, i.e. currently silicon nitride, with a material that has the same or a very similar optical density as or to the material of the intermetal dielectric used, copper. However, this would require a redevelopment and qualification of the entire back-end process.
Furthermore, one possibility for solving the problem described above would be to irradiate the integrated layer stack arrangement in accordance with the prior art with light having increased intensity in order thus to compensate for the only low transmission. However, this procedure considerably increases the current consumption, thereby decreasing for example the battery endurance time of radio mouses, for example. Generally, an increased current consumption in the case of a device with such an integrated layer stack arrangement is not desired.